Improvements of the Driver of ORCHIDEE

These improvements will concern first the specifities of the driver (its capacity to read forcing files), not its performances that will be improved hopefully by the use of the IOServer. The focus is mainly on the use of forcing files at a 6 or 3-hourly time resolution (such as re-analysis), for which the driver interpolates the meteorological fields to calculate half-hourly fields.

Reminder

The driver of ORCHIDEE is expected to read forcing files with a very specific temporal structure. All time information are defined on UTC time.

The fields read at a certain time step t in the NetCdf? file correspond to:

the instantaneous value at t+1 for Temperature, Wind Speed, Air Pressure and Specific Humidity. the mean value between t and t+1 for Long and Short Wave Incomming Radiation and the Precipitation.

• Interpolation
  • Instantaneous fields and the Long-Wave Radiation are then linearly interpolated for each t:t+1 time period in order to calculate half-hourly value.
  • The Precipitation field is downscaled to half-hourly resolution using a step distribution function with one parameter (nb_spread, that is the number of half-hourly time steps over which we spread the precipitation). nb_spread can vary from 1 (all precipitations over a forcing time period (3 ou 6 hours) are put on the first time step) to SPLIT_DT (number of integration time steps with a forcing time period, in that case, the precipitation are distributed uniformely). By default, nb_spread is set to 1.
  • The Short-Wave radiation is interpolated using a function distribution that corresponds to the solar angle distribution over a forcing time period.

Add an explicit time information within the forcing files

As described above, the driver of ORCHIDEE is relatively rigid because it only assumes one temporal specification for the forcing files. At first, we do not expect to develop a more flexible driver but we would like to add explicit time information in the driver in order to avoid misuse. We suggest to add time_bounds attributes to the variables stored in the NetCdf? forcing files.